Inconsistent focus...Is it my lens or camera? Is this normal? (with test pics)

[realityinabox]

I am currently in the process of upgrading my equipment on a budget, so I am trying to figure out where my limitations are. I have been getting inconsistent results from my photos, some soft, some sharp, so I wanted to pinpoint the cause.


I decided to run a little test. I mounted my Rebel XT onto a tripod and aimed it directly at a piece of sheet music, testing both my lenses (Sigma 28-70 f/2.8 and Canon 50mm f/1.8). With the Sigma, I got various results over the range of the barrel, which is what I expected. Most focal lengths at anything less that f/5.8 were terribly soft.


I noticed that I wasn't getting consistent sharpness across pictures at the same settings, so I threw my Canon 50mm on there to see if it was just the crappy Sigma giving me trouble.


At f/1.8 the shots were soft, but to varying degrees. I was able to get a few sharp shots at f/2.8 and f/4, but not consistently.


Here are the results from the Canon 50mm f/1.8. Taken from a tripod, one after the other. 100% crops of the center of the frame (lets not even talk about the edges):


f/2.8:





f/4:





Is this much variation in sharpness normal?


If not, would you say it is my camera (the variations happens with both my Sigma and Canon lens), or my lenses (both are cheap, could just be a problem with their AF)?


I want the get the best bang for my buck on upgrading, but I don't want to buy a $1k lens if it is going to give me inconsistent results because something is wrong with my body, and vice versa.





UPDATE: I just tried the exact same setup with my wife's Nikon D40 and her kit 18-55mm at 55mm. 6 shots, all sharp, hardly anyperceivabledifference between them. Now I know something is up!

[Sean Setters]

Do you know anyone else who shoots Canon and can let you use another (higher quality) lens for testing purposes? That might help you narrow down the possible causes of focus variation...

[JamesBong]

Is the sheet music at a right angle to the lens direction?


Is the focus even across the image?


Are you using all of your focus points, or just the center?

[realityinabox]

Is the sheet music at a right angle to the lens direction?

Yes.

Is the focus even across the image?


Are you using all of your focus points, or just the center?

I had it set on auto-select at first. Every time I re-clicked to focus, a different configuration would appear. I would re-click multiple times in a row to try to get the most points in focus, but it was never the same ones and every time I would click, I could hear/see the lens refocusing slightly.


It was at this point that I switched to selecting only the center focus point. Again, every time I would click to focus, I could hear it refocus slightly. I posted the results above, obviously not consistent.


Both lenses did this. The best way I can describe it is a slipt second stutter where the focus goes out then back in, butapparentlydoesn't find the correct focus every time (or even most of the time).

[Sean Setters]

How much ambient light was in the room? Also, whenever calibrating a microfocus adjustment, you're advised to switch to manual focus and set the focus to infinity, and then switch the autofocus back on so that the focus has to go all the way back to find the correct focus point. You might try that--just so that the focus motor has to work for each individual shot--and, of course, use the center point.


Also, you might want to try setting the focus sheet at a 45 degree angle (or, rather, your camera at a 45 degree angle if your paper is taped to a wall). That way you can see if the lens is consistently front or back focusing.

[realityinabox]

Thanks for those tips.


I just ran some more test shots right now, enough ambient light to shoot 1/30 f/2.8 ISO 400. I tested both lens, shot 8 shots with both, using the 45 degree angle and with the manuel to infinity trick.


15 out of 16 of the shots were front focused. One (from the 50mm) was sharp at the center. The others were sharp at about 1/3 up the frame.


So what now?


Update: Upon further testing, the Sigma seems to be front focusing consistently, and the Canon 50mm front focuses about 50% of the timeuntil f/4, where it locks on center thereafter.

[scalesusa]

It sounds like both may have problems, but it could also be the camera body itself. If the lenses are in warranty, send them in for repair. Otherwise, the problem will not go away, even if you try microfocus adjust.


I bought a used 50mm 1.4 which was erratic, Canon checked it and found a broken AF mechanism. once repaired, it was right-on all the time. I'be never owned the Sigma, but it does have a reputation for AF problems.